The invention relates to a four-wheel vehicle drive system and more specifically to an adaptive vehicle drive system which incrementally shifts from two-wheel drive to four-wheel drive upon sensing certain conditions.
The performance advantages of four-wheel vehicle drive systems are well recognized. Improved vehicle stability while traversing rain soaked or ice or snow covered highways, handling and control on gravel or uneven pavement and simply maintaining traction in off road situations are all readily acknowledged benefits. Concomitant and less desirable attributes of four-wheel drive systems relate to reduced gas mileage from increased drive line friction and increased vehicle weight. Such increased drive line friction occurs in part time four-wheel drive systems which rotationally couple the front and rear vehicle propshafts. Such vehicle weight increases are particularly pronounced if the system is designed with a differential between the front and rear drive shafts for full-time engagement and operation rather than intermittent operation when conditions specifically demand it.
Furthermore, while part time four-wheel drive systems which lock the front and rear propshafts together provide obvious benefits of traction and stability in straight line driving, the disparity between the ground speed at the front wheels and the ground speed at the rear wheels during cornering can itself result in wheel slip and hopping of the vehicle. Thus, allowing the front and rear output shafts of the transfer case to operate at different speeds during cornering is beneficial.
Many four-wheel drive systems employing diverse control and torque distribution strategies have been designed and utilized. These various approaches are embodied in United States patents.
For example, U.S. Pat. No. 4,417,641 teaches an automatic four-wheel drive system having an electromagnetic clutch and steering sensor. When the steering wheels are turned greater than a predetermined angle, the electromagnetic clutch is de-energized and two of the driving wheels are disconnected. The system also includes a slip sensor.
U.S. Pat. No. 4,718,303 is co-owned by the assignee herein. It discloses a transfer case having an electromagnetic ramp clutch which is modulated to adjust the torque distribution in a full time four-wheel drive system.
In U.S. Pat. No. 4,937,750, a microcomputer compares signals from front and rear axle speed sensors. If the difference is greater than a certain value, a clutch is engaged to interconnect the front and rear prop shafts to prevent slip.
U.S. Pat. No. 4,989,686 discloses a full time four-wheel drive system including wheel slip detectors. The output signal from the wheel slip detectors is utilized to drive a proportional clutch which then delivers torque to whichever axle is rotating more slowly. U.S. Pat. No. 5,002,147 discloses a four-wheel drive system which achieves torque splitting between the front and rear axles. The system utilizes four separate wheel speed sensors as well as a steering angle sensor.
In U.S. Pat. No. 5,060,747, a vehicle torque distribution system is taught which includes means for determining both vehicle speed and the difference between the speeds of the front and rear wheels which indicates wheel slip. The vehicle speed data is utilized to correct the sensed wheel speed difference and this corrected value is then utilized to produce a clutch engagement signal.
U.S. Pat. No. 5,090,510 discloses a four-wheel drive system having a differential and a hydraulic clutch disposed in parallel between the front and rear drive shafts. Engagement of the clutch restricts action of the differential to achieve a desired rotational speed difference between the front and rear wheels.
The foregoing review of relevant United States patents reveals many approaches to controlling torque delivery in four-wheel drive vehicles and certain shortcomings as well. Hence, improvements in the art of four-wheel drive vehicle torque delivery systems are both possible and desirable.